The goal in today's aviation market is to improve airport capacity. Heavy aircraft produce very strong vortices when flying at low speed during takeoff and landing, thus creating potential hazards for following aircraft. Consequently, the wake vortex hazard has become the limiting factor in airport capacity.
Flying aircraft leave a system of vortices trailing behind as a necessary consequence of the generation of lift. For large transport aircraft, these vortices typically coalesce into a pair of counter-rotating vortices. The strength of the vortices is proportional to the weight of the aircraft and inversely proportional to the aircraft speed. Therefore, a heavy aircraft produces very strong vortices when it is flying at low speed during takeoff and landing. Under turbulent atmospheric conditions, trailing vortices are diffused rapidly and pose little hazard. However, when the atmosphere is calm or weakly turbulent the vortices can pose a major hazard. The vortices are strong enough that they can cause loss of control and/or physical damage to following aircraft, where several accidents have been attributed to the wake vortex hazard. Currently, to manage this problem large longitudinal spacing between aircraft and large lateral spacing between parallel runways are required to avoid any chance of a wake vortex being encountered by a trailing aircraft. As a result of these rules, the wake vortex hazard has become the limiting factor in airport capacity.
Two counter-rotating vortices eventually interact with one another, causing them to link and destroy one another. This process is caused by instabilities and is well known. The problem is that these instabilities build up slowly, and under natural conditions it takes a long time and distance for the instability to lead to vortex destruction.
The concept of causing early wingtip vortex destruction by exciting instabilities is of paramount interest. Investigation of several different “perturbators” that produce disturbances in the vortex core has been documented. A Kármán vortex street occurs naturally whenever flow passes over a bluff body at sufficiently high Reynolds number. The Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over bluff bodies. A bluff body such as a cylinder or a flat plate will periodically shed alternating sign vortices. When a vortex is shed, an unsymmetrical flow pattern forms around the wing, which therefore changes the pressure distribution. This means that the alternate shedding of vortices can create periodic forces on the wing surface, causing it to vibrate. If the vortex shedding frequency is similar to the natural frequency of a wing, it causes resonance. With aircraft this periodic force is highly undesirable. These vortices are perpendicular to the flow direction, and thus also perpendicular to the wingtip vortex. Passive, fixed-position vortex shedding perturbators mounted near the trailing edge of wings, such as a bluff body or a transverse jet, that are positioned in the flow have been studied to determine if the Kármán vortex street will be embedded in the tip-vortex core to cause periodic perturbations to the tip vortex, thus exciting natural instabilities of the vortex system for early destruction. By choosing the size of the bluff body correctly, the perturbations will be the correct wavelength to excite instability in the tip vortex core to make the vortex core diameter larger. These perturbators attached to wings that have two or more lifting surfaces were determined to have a high parasitic drag when deployed and limited to exploiting instability of only a single vortex core, causing that core to increase in size. Having a bluff body protrude into the flow to induce disturbances will cause two problems. First, it will increase the drag of the wing significantly. This may be of less concern if it is intended to be deployed only during takeoff and landing. However, takeoff requires maximum engine thrust and is hindered by the added drag. Secondly, by its very nature the bluff body perturbator will cause vibrations of the wing and the entire aircraft, which may have similar unwanted resonance consequences as mentioned above.
Accordingly, there is a strong need to develop new ways to minimize wake vortex hazards for reducing the large longitudinal spacing between aircraft and large lateral spacing between parallel runways to avoid any chance of wake vortices being encountered by trailing aircraft and thus improve airport capacity
What is needed is a way to produce rapid variations in the position of each vortex. Rapid variations will cause the vortex emanating from each wing to oscillate resulting in interaction between the two vortices and subsequent destruction much earlier than it would occur naturally.